Aeroelastic tuned mass damper

ABSTRACT

A method for damping aeroelastic modes, including limit cycle oscillations (LCO), is implemented by determining a mass for a tuned mass damper (TMD) based on an modal frequency for a mode having a potentially positive growth rate and attaching a TMD to at least one attachment point with significant motion such that a damping axis of the tuned mass damper is substantially oriented in a direction aligned with the local modal deflection.

BACKGROUND INFORMATION

1. Field

Embodiments of the disclosure relate generally to the field of vibrationreduction systems for aircraft and more particularly to a plurality oftuned mass dampers having viscous damping and mounted in multiplelocations on the airframe with directional orientation determined tomaximize damping of primary modes of aeroelastic limit cycleoscillation.

2. Background

Large modern commercial jet aircraft are designed with consideration ofthe aeroelastic stability of the aircraft. However, in certain casesaeroelastic designs may be subject to resonant oscillations createdunder certain aerodynamic conditions and at various speeds. Suchoscillations can be localized in certain portions of the airframe or maybe whole airframe aeroelastic modes including limit cycle oscillations(LCO) involving the nacelles, wing and fuselage.

To minimize LCO, prior art aeroelastic solutions include payload and/orfuel restrictions, active modal suppression using control surfaces,adding ballast, vortex generators to change aerodynamic flowcharacteristics and structural changes (such as adding wing stiffness).Payload or fuel restrictions will typically reduce capability of theaircraft while active modal suppression requires extensive design andexperimentation resulting in extended design lead time and may alsoaffect performance. Use of ballast results in a significant increase inweight which may affect performance and may drive structural changes andinherent structural changes for stiffness also typically add weight.Vortex generators, while often effective for localized oscillationsuppression are not typically effective for full airplane LCO

It is therefore desirable to provide modal damping to satisfyaeroelastic stability and vibration requirements with low cost, simpledesign elements with minimized weight increase and no performanceimpact.

SUMMARY

Embodiments disclosed herein provide a method for damping aeroelasticmodes including whole airframe limit cycle oscillations (LCO)implemented by determining a mass for a tuned mass damper (TMD) based ona LCO aeroelastic mode frequency having a potentially positive growthrate and attaching a TMD to an attachment point such that a damping axisof the tuned mass damper is substantially oriented in a directionaligned with the modal deflection at a location having significantmotion.

In an example embodiment for the TMD, a tuned mass assemblyincorporating a primary mass and tuning masses is concentrically mountedon a shaft with opposing concentric springs with a viscous damper forthe tuned mass. The viscous damper includes magnets mounted on the tunedmass assembly and a case having a conductive, non-magnetic, surfacemounted concentrically to the shaft adjacent the magnets for generationof eddy currents. The shaft is supported by end caps mounted to theaircraft attachment point. In one embodiment, one or more TMDs aremounted in aircraft nacelles for reciprocation on an axis oriented in aninboard and outboard direction.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments of the present disclosureor may be combined in yet other embodiments further details of which canbe seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic pictorial view of a TMD for aeroelastic modedamping;

FIG. 2 is a detailed schematic cutaway of the TMD of FIG. 1;

FIG. 3A is a rear right isometric view of an example embodiment of anadjustable TMD confirmation tool as mounted on the nacelle inletbulkhead;

FIG. 3B is a rear left isometric view of the TMD of FIG. 3A with theouter case removed;

FIG. 3C is a partial exploded view of the TMD of FIG. 3A;

FIG. 4 is an isometric view of the primary mass;

FIG. 5 is an isometric view of the primary mass assembly;

FIG. 6 is an isometric view of the translating mass buildup;

FIG. 7A is an isometric view of the left end cap;

FIG. 7B is an isometric view of the right end cap showing the shaftcapture bushing;

FIG. 8 is an end view of the TMD as installed showing the support bladeand link assembly;

FIG. 9 is a rear section view of the nacelle bulkhead with the TMDinstalled;

FIG. 10 is a side partial section view of the nacelle showing the TMDlocation as mounted and the section view line FIG. 9-FIG. 9;

FIG. 11 is a detailed rear view of the installed TMD from view ring

FIG. 11 shown in FIG. 9;

FIG. 12 is a bottom view of the installed TMD;

FIG. 13 is a top view of an example aircraft in which the TMD isinstalled for LCO suppression;

FIG. 14 is a graph of aeroelastic growth rate vs. speed with suppressionby the TMD illustrated;

FIG. 15 is a graph of the increase in aeroelastic damping with respectto viscous damping in the TMD;

FIG. 16 is graph of flutter mode damping for three selected TMD masses;and,

FIG. 17 is a flowchart of the method for whole airframe limit cycleoscillation damping using the TMD embodiments described.

DETAILED DESCRIPTION

Embodiments disclosed herein provide a tuned mass damper (TMD) to dampenaeroelastic modes involving the complete primary structure includingwhole airframe limit cycle oscillation (LCO) vibration involving thepowerplant, wing and fuselage. In an example embodiment, the tuned massdamper is attached to the nacelle in the region of the lower fan case.In alternative embodiments, the damper may be attached to one or more ofthe airplane nacelles, (or other locations on the airframe). For theembodiment described in detail subsequently, the TMD is located in thelower forward nacelle inlet cowl in a horizontal position with inboardand outboard motion of the mass to maximize effectiveness for anairplane LCO mode which has significant displacement at this locationand direction. The TMD frequency is equal to the modal frequency forwhich suppression is desired. The mass is mounted to move with minimalCoulomb friction and is provided with an optimized amount of viscousdamping (proportional to velocity of the mass in the TMD and ˜2 to 5%for the example embodiment). The viscous damping may be obtained bymeans of pneumatics, hydraulics, or as in this embodiment, magneticbraking.

FIG. 1 shows a production TMD 2 as implemented for a particularlydefined aeroelastic mode and direction. The TMD 2 is mounted to an inletbulkhead 3 in an engine nacelle for a large commercial aircraft. TMD 2incorporates a primary mass 4 mounted on a shaft 5. The mass 4 isconstrained by springs 6 for reciprocal motion with the combination ofweight of the mass and the spring constant of the springs defining atuned frequency. Viscous damping is achieved with a magnetic element 7which reacts with a conductive surface 8 adjacent and parallel to thedirectional motion of the mass creating eddy currents. In alternativeembodiments, hydraulic or pneumatic systems associated with the mass forcreation of the desired viscous damping.

The overall position of the TMD 2 in the aircraft 1 is shown in FIG. 2with relative positioning of the inlet bulkhead 3 and showing theinboard and outboard direction of oscillation of the TMD with arrow 9

As shown in FIG. 3A for an example embodiment, a configurationconfirmation tool TMD 10 is mounted to an inlet bulkhead 3 in an enginenacelle for a large commercial aircraft. The features described hereinprovide a test and evaluation tool for confirmation of theconfiguration, sizing, damping and orientation requirements to achievedesired aeroelastic modal damping. A case 14 houses the TMD operatingcomponents and various brackets are employed to mount the TMD to thebulkhead as will be described in greater detail subsequently. As shownin FIGS. 3B and 3C, the TMD 10 includes a translating mass buildup 16which is supported for reciprocal oscillation on a shaft 18. Orientationof the shaft establishes a damping axis 19 for the TMD. Springs 20,concentric to the shaft and constrained with inner spring seats 22 andouter spring seats 24, resiliently constrain the translating massbuildup 16 for resonant motion response. The shaft 18 is supported byleft and right end caps 26 which mount to the bulkhead 3. Case 14 hastwo separable halves 14 a and 14 b which include slotted reliefs 28which allow upper and lower accelerometer posts 30 to protrude.

The translating mass buildup 16 includes a primary mass 32 shown in FIG.4 which is employed in a primary mass assembly 33 shown in FIG. 5. Theprimary mass 32 has a central boss 34 with symmetrical cylindricalextensions 36. The central boss includes flats 38 for mounting of theaccelerometer posts 30. Additionally, the central boss may includemachined weight adjustment pockets 39. Flanged grooves 40 in the centralboss receive magnetic rings 42 for damping to be described in greaterdetail subsequently. For the embodiment shown, the magnetic rings aresemi-cylindrical halves 42 a and 42 b joined with screws 43 for mountingwithin the flanged grooves 40. Shoulders 44 on the primary mass receivethe inner spring seats 22. The primary mass 32 has a central axial bore46 which incorporates a low friction bearing 48 receiving the shaft 18(as seen in FIGS. 3B and 3C).

The primary mass assembly 33 includes the primary mass 32 with magneticrings 42 mounted in the flanged grooves 40. Interconnecting halfcylindrical ring magnet spacers 50 constrain the magnetic rings in theflanged grooves and provide physical spacing of the magnets from thecase halves 14 a and 14 b in which the translating mass builduposcillates. Additionally, outboard faces 52 of shoulders 44 incorporatethreaded bores which receive studs 54.

The translating mass buildup 16 is shown in detail in FIG. 6 includestuning masses 56 a, 56 b, 56 c and 56 d which are removably mounted onthe studs 54 concentrically over the cylindrical extensions 36 on eachside of the primary mass. The tuning masses in the configurationconfirmation tool version of the TMD provide adjustment for exactfrequency matching in the TMD to the desired frequency of theaeroelastic mode to be damped. Lock washers 58 and jam nuts 60 securethe tuning masses to the studs.

FIGS. 7A and 7B show the end caps 26 which support the shaft 18 andmount the TMD to the aircraft nacelle inlet bulkhead. Bores 62 inlateral flanges 64 receive threaded ends 66 of the shaft 18 (as shown inFIG. 3C). As shown in FIG. 7B bushings 68 are inserted in the bores 62to closely receive the shaft ends 66 which are constrained by nuts 69and associated washers 70 (also seen in FIG. 3). Transverse brackets 71extend from the lateral flanges 64 for mounting to the bulkhead. As seenin FIG. 7B, a raised disc 72 on inner surfaces 74 of the end cap receiveand locate the outer spring seats 24 (as best seen in FIG. 3).

Mounting of the TMD employing the end caps is shown in FIG. 8.Transverse brackets 71 are attached to a strengthening plate 76 usingfasteners 78. The plate 76 then mounts to bulkhead 3. Additionalstability of the TMD is provided through blades 80 which extend from andare attached to the lateral flanges 64. Attachment of the blades 80 tolink fittings 82 with link assemblies 84 provides torsional stabilityfor the cantilevered TMD.

Details of the location and orientation of the TMD mounting for theexample embodiment are shown in FIGS. 9-13. FIG. 9 shows the inletbulkhead 3 as a section view FIG. 9-FIG. 9 in the engine nacelle 90 seenin FIG. 10. For the embodiment shown, the TMD mass translates inboardand outboard with respect to the aircraft as represented by arrow 92 inFIGS. 9 and 11. The TMD is a damped resonant oscillator with theresonant frequency established by the total mass of the translating massassembly 16 and the spring constants of the springs 20. Very precisetuning of the resonance can be achieved by variation of the tuningmasses 56 a-56 d previously described. Viscous damping is accomplishedfor the embodiment shown by means of eddy currents developed by magnetrings 42 attached to the moving translating mass assembly 16 close to astationary conductive metal surface of the case 14. The magnet rings arereplaceable in the mass buildup for altering the eddy currentinteraction with the conductive surface to adjust the viscous dampinglevel. For the described embodiment viscous damping of approximately2-5% is achieved. In alternative embodiment, damping could be achievedby fluid flow or other means.

As seen in FIG. 11, the TMD with end caps 26 is mounted to strengtheningplate 76 which is attached to the bulkhead 3. For the exampleembodiment, secondary retention of the TMD under destructive loadconditions that might result in expulsion of the TMD from the nacelleinlet is provided by retention cables 94 which attach to the blades 80on each side of the TMD and are routed to retention fittings 96connected to the bulkhead 3. Additionally, cabling 98 for electricalconnection to accelerometers mounted in the accelerometer posts 30 isrouted through wire brackets 100.

FIG. 13 shows the mounting location of the TMDs of the exampleembodiment in the nacelles 90 extending from the wings 101 of an exampleaircraft 102. For an aircraft in which whole airframe LCO withsignificant lateral (inboard/outboard) motion of the nacelles ispresent, the TMD of the described embodiment has been demonstrated toeffectively reduce the growth rate of chosen aeroelastic modes andprovide sufficient aeroelastic modal damping for acceptable aircraftflight characteristics.

For the example aircraft, the aeroelastic mode of interest is shown inFIG. 14. The flutter mode at a principal resonant frequency, trace 120,shows potential positive growth rates in the operating airspeed regime122 and required reduction for acceptable aircraft performance.Implementation of the TMD as described for the embodiment disclosedprovided a significant improvement in the aeroelastic mode growth rate124 as shown in trace 126 in FIG. 14.

Adjustment of the viscous damping in the TMD allows enhancement of theflutter mode damping as shown in FIG. 15. Curve 128 shows increasingflutter mode damping over a range of between 0.04 to 0.28 g with amaximum increase in the flutter mode damping at approximately 0.11 gviscous damping.

FIG. 16 shows the flutter mode damping provided for various masses inthe TMD of 75 lbs, trace 130, 100 lbs, trace 132 and 150 lbs, trace 134.A 100 lb mass provides an acceptable damping level over a full range ofoperating mass damper viscous damping values of 0.05 to 0.5 g.

A method for adding aeroelastic damping by employing the embodimentsdescribed herein is shown in FIG. 17. Critical aeroelastic modes withpotential undesirable growth rates are determined, step 1700, andlocations/directions that have significant modal deflections areindentified, step 1702. An initial mass and stroke for at least onetuned mass damper is determined, step 1703. Using a TMD configurationconfirmation tool, the mass is adjusted with tuning masses, step 1704.The tuned mass assembly is then concentrically mounted on a shaft forreciprocal motion, step 1706, constrained by opposing springs, step1708. Removable viscous damping magnets are attached to the tuned massassembly, step 1710, and a metallic case is attached concentricallysurrounding the tuned mass assembly and magnets for eddy currentgeneration, step 1712. Ends of the shaft are received in end caps tocreate the TMD, step 1714, and the end caps are secured in a directionaligned with modal deflection at a location having significant modaldeflection, step 1716. One TMD attached to each nacelle inlet bulkheadon the aircraft with inboard/outboard orientation of the shafts toestablish the damping axis for reciprocal oscillation of the masses areemployed for the embodiments disclosed. Support for cantilever loads onthe TMD may be added, step 1718, and assembly lanyard retention cablesto avoid expulsion of the TMD may be attached, step 1720. Uponconfirmation of the desired damping of LCO by the configurationconfirmation tool TMD, a production TMD with a mass from the optimumtuned mass and optimized viscous damping of the configurationconfirmation tool is defined, step 1722. The production TMD is thenmountable at the locations and orientations determined by theconfiguration confirmation tool TMD for LCO damping, step 1724.

Having now described various embodiments of the disclosure in detail asrequired by the patent statutes, those skilled in the art will recognizemodifications and substitutions to the specific embodiments disclosedherein. Such modifications are within the scope and intent of thepresent disclosure as defined in the following claims.

What is claimed is:
 1. A method for damping aeroelastic modes, includinglimit cycle oscillations (LCO), comprising: determining a mass for atuned mass damper (TMD) based on an aeroelastic mode having apotentially positive growth rate; and, attaching at least one TMD to atleast one attachment point such that a damping axis of the tuned massdamper is substantially oriented in a direction aligned with modaldeflection at a location having significant modal deflection.
 2. Themethod for damping aeroelastic modes as defined in claim 1 wherein thestep of attaching at least one TMD to at least one attachment pointcomprises attaching a TMD to an inlet bulkhead of at least one enginenacelle.
 3. The method for damping aeroelastic modes as defined in claim2 wherein the step of attaching a TMD to an inlet bulkhead of at leastone engine nacelle comprises attaching a TMD to the inlet bulkhead ofeach engine nacelle in the aircraft.
 4. The method for dampingaeroelastic modes as defined in claim 1 further comprising employing aconfiguration adjustment TMD for adjusting the mass with tuning massesfor a tuned mass assembly.
 5. The method for damping aeroelastic modesas defined in claim 4 further comprising concentrically mounting thetuned mass assembly on a shaft for reciprocal motion.
 6. The method fordamping aeroelastic modes as defined in claim 5 further comprisingconstraining the tuned mass assembly with opposing springs.
 7. Themethod for damping aeroelastic modes as defined in claim 6 furthercomprising attaching viscous damping magnets to the tuned mass assemblyand attaching a non-magnetic, conductive metal case concentricallysurrounding the tuned mass assembly and magnets for eddy currentgeneration.
 8. The method for damping aeroelastic modes as defined inclaim 7 further comprising supporting the TMD in a cantilevered positionfrom the bulkhead.
 9. The method for damping aeroelastic modes asdefined in claim 8 further comprising constraining the TMD with alanyard cable.
 10. The method for damping aeroelastic modes as definedin claim 1 wherein the direction aligned with the modal deflection isinboard and outboard.
 11. A method for configuration confirmation fordamping aeroelastic modes with a TMD having a tuned mass assemblymounted on a shaft for reciprocal motion constrained with springsconcentric to the shaft and viscous damping magnets on the tuned massassembly with a metallic case concentrically surrounding the tuned massassembly and magnets for eddy current generation comprising: determininga mass for the TMD based on a LCO flutter mode frequency having apotentially positive growth rate; adjusting the mass with tuning massesfor a tuned mass assembly; attaching a TMD to an inlet bulkhead on eachnacelle of an aircraft such that a damping axis of the tuned mass damperis substantially oriented in a direction aligned with modal deflection;supporting the TMD in a cantilevered position from the bulkhead; and,constraining the TMD with a lanyard cable.
 12. An aeroelastic modaldamping system comprising: at least one tuned mass assembly mounted toan aircraft attachment point for reciprocation on an axis oriented in aninboard and outboard direction; and, a viscous damper for the tunedmass.
 13. The aeroelastic modal damping system as defined in claim 12wherein the tuned mass assembly comprises: a primary mass and adjustabletuning masses concentrically mounted on a shaft with opposing concentricsprings, the shaft supported from the attachment point by end caps. 14.The aeroelastic modal damping system as defined in claim 13 wherein theviscous damper comprises: at least one magnet mounted on the tuned massassembly; and a case having a metallic surface mounted concentrically tothe shaft adjacent the magnet for generation of eddy currents.
 15. Theaeroelastic modal damping system as defined in claim 13 wherein theattachment point comprises an inlet bulkhead of a nacelle.
 16. Theaeroelastic modal damping system as defined in claim 15 furthercomprising blades extending from the end caps and connect with linkassemblies to the inlet bulkhead for cantilever support.
 17. Theaeroelastic modal damping system as defined in claim 15 wherein the atleast one tuned mass assembly comprises a plurality tuned massassemblies, one of said plurality mounted on each of a plurality ofnacelles on an aircraft.
 18. An aircraft with an aeroelastic modaldamping system comprising: a plurality of nacelles extending from thewings of an aircraft; a plurality of TMDs each having a tuned massassembly with a primary mass concentrically mounted on a shaft withopposing concentric springs, the shaft supported by end caps; and, aviscous damper for the tuned mass having at least one magnet mounted onthe tuned mass assembly and a case having a metallic surface mountedconcentrically to the shaft adjacent the magnet for generation of eddycurrent; each TMD mounted to a respective one of the nacelles with theshaft on an axis oriented for reciprocation in an inboard and outboarddirection.